1. Why does the QuEChERS approach recommend use dry ice or liquid nitrogen homogenizing frozen samples and extracting samples while frozen? The frozen samples were homogenized with dry ice or liquid nitrogen, and the samples were made into a fine powder (smaller particle size, more homogeneous), thus maximizing the extraction surface area and making the samples easier to process. Samples should be extracted during freezing to prevent analyte degradation.
2. When I used the QuEChERS method to reconstruct the extract in LC/MS mobile phase solvent, precipitation would occur. How do I get over it? If sediment is present during reconstruction, it can be ultrasonically treated and centrifuged to see if the sediment has entered the solution. If the recovery solvent is a percentage of methanol in water, first add methanol to dissolve the sample. Ultrasonic treatment and vortex before adding water.
3. Why wasn’t the original QuEChERS approach buffered? The non-buffered was developed for the analysis of fewer pesticides. The buffered is introduced, to apply to larger pesticide screen, and many compounds show pH dependence.
4. In the extraction procedure of the QuEChERS method, what are the AOAC criteria for adding extractive salts and solvents? For each gram homogeneous sample, it is recommended to add 1 ml 1% acetic acid to acetonitrile and 0.5 g anhydrous magnesium sulfate/sodium acetate (4/1, w/w) to acetonitrile as required by AOAC 2007.01.
5. In the dispersion d-SPE step of the QuEChERS method, what are the AOAC criteria for adding magnesium sulfate and SPE adsorbent? For each 1 ml sample in the extraction step, a ratio of 3:1 magnesium sulfate/SPE adsorbent mixture of 200 mg is recommended. For example, it is recommended to use 1200 mg of magnesium sulfate and 400 mg of PSA when the 8 ml bisected sample in the extraction step is subjected to the dispersed SPE purification step.
6. In the QuEChERS method, how many (volume) of the initial extract and dSPE extract do I expect after centrifugation? Depending on the moisture content of the sample, 15 g of the sample produces approximately 11-14 ml of the initial acetonitrile extract. During the SPE step, the salt and adsorbent dSPE blend lost almost half of the extract, with approximately 6 ml of the final extract extracted from the 15 g sample.
7. What modifications can you make to dry samples to improve the extraction procedure of the QuEChERS method? For dry samples with less than 80% water content, you can start with half of the sample and then add cold water to bring the AOAC method to 15 g and EN method to 10 g.EN 15662 provides specific guidelines for adding water to different substrates. For example, it is recommended to add 10 ml of water to a 5 g grain sample.
8. Using the QuEChERS extraction kit, I accidentally sprinkled some salt while distributing it into the centrifuge tube. Will my results be affected? QuEChERS extracted the excess salt in the procedure. Salt was first added to remove some of the water and cause the phase separation between acetonitrile and water. If the overflow (<1g) does not affect your results.
9. Is it necessary to dry and reconstruct the extract from the d-SPE step in the QuEChERS method for LC/MS analysis? The chromatographic properties can be improved by evaporating the sample and pre-preparing it in the initial LC mobile solvent. Strongly recommended, but not required. If the analyte of interest is not eluted early in the chromatogram, you can leave without separating the sample or drying.
10. What is the approximate pH to be achieved using citrate as per EN 15662? What is the buffer range? Most samples have a pH of between 5 and 5.5. Buffer range is approximately pH 2-7.4.